get_exit_type checks and converts a vector x
of FFT exit descriptions into exits of an FFT
that correspond to the current options of
exit_types (as a global constant).
Details
get_exit_type also verifies that the exit types conform to an FFT
(e.g., only the exits of the final node are bi-directional).
See also
FFTrees for creating FFTs from and applying them to data.
Other utility functions:
get_best_tree(),
get_fft_df()
